1. Field of the Invention
This invention relates to an optical memory element which optically records information therein.
2. Description of the Prior Art
In recent years, an optical memory system has been developed, which optically stores information in high density and in a mass storage order. The optical memory system ensures high density and mass storage because the information recording unit (bit) is determined by the diameter of the beam which is used in the recording operation and the reading operation. Generally, the bit size is selected to be about 1 .mu.m. However, in order to have the optical memory system ensure high density and mass storage, the optical beam position must be accurately controlled so that the information is recorded at a predetermined position and the information is read out from a preselected position. With the optical memory system of a read only type, in general, address information can be recorded on the optical memory element when data information is recorded on the optical memory element. Accordingly, the optical beam position is controllable while the data information is read out from the optical memory element. However, in the optical memory system of an add-on-memory type or an erasable memory type, it is difficult to record the address information on the optical memory element while the data information is recorded on the optical memory element. Therefore, in the add-on-optical memory or the erasable optical memory, guide signals or guide addresses are normally recorded on the optical memory element, which are used to control the optical beam position.
FIG. 3 schematically shows the conventional optical memory element of an add-on-memory type (e.g., a DRAW type disc using TeOx, etc., as a recording material) or an erasable memory type (e.g., an optically magnetic disc using an alloy including rare earth elements and transition metals as a recording material), in which striped grooves are formed on the substrate of the optical memory element. The information recording operation and the information playback operation are conducted along the striped grooves. Some methods, typically three kinds of methods, for the formation of the striped grooves on the substrate of an optical memory element have been proposed as follows:
(1) A method for the formation of the abovementioned grooves on the substrate, in which a mold having striped grooves is employed to transcribe the striped grooves onto a resin substrate made of acrylic resin or polycarbonate resin by an injection molding technique. PA1 (2) A method for the formation of the abovementioned grooves on the substrate, in which a mold having striped grooves is employed to transcribe the striped grooves onto a resin substrate made of acrylic resin or polycarbonate resin by a casting technique. PA1 (3) A method for the formation of the above-mentioned striped grooves on the substrate, in which UV-setting resin is interposed between the stamper having striped grooves therein and the substrate made of acrylic resin, epoxy resin, glass, etc., and the interposed resin is then exposed to UV rays through the substrate so as to be cured, followed by removing the stamper. This method is known as "the 2P method".
Since all of the three methods use a resin layer, there is a possibility that oxygen or moisture will reach the recording material through the resin layer, which will cause deterioration of the recording material disposed on the substrate. That is, the conventional method does not ensure stable operation of the optical memory element. Especially, in optically magnetic memory elements in which a rare earth metal-transition metal alloy film made of GdTbFe, TbFeCo or the like is used as a recording material, it is undesirable to employ the above-mentioned resin layer for the substrate since the rare earth metals such as Gd, Tb, etc., are extremely active elements.
In order to eliminate the above-mentioned problems, the inventors of this patent application have proposed a method for the formation of striped grooves on a glass substrate by U.S. Pat. No. 4,544,443, in which a photo-resist material is coated on the glass substrate to form a photo-resist film; UV rays are applied to the photo-resist film through a photo-mask to form a guide groove pattern latent image on the photo-resist film; the said guide groove pattern latent image is developed; and then an etching operation is conducted through the said developed guide groove pattern so as to form the guide grooves in the glass substrate. However, the smoothness of the surface of the glass substrate is significantly damaged by the etching operation, which causes an increase in noise.